The invention relates to a method of controlling release of a hoisting motor brake in a hoisting apparatus, where electricity is used as the driving force and a squirrel cage motor as the hoisting motor for hoisting or lowering a load attached to a hoisting member of the hoisting apparatus.
The invention further relates to an apparatus for controlling release of a hoisting motor brake in a hoisting apparatus, where electricity is used as the driving force and a squirrel cage motor as the hoisting motor for hoisting or lowering a load attached to a hoisting member of the hoisting apparatus.
In electric hoisting apparatuses intended for hoisting and lowering a load the hoisting motor includes a brake by means of which the load to be hoisted or lowered is kept in the air when the hoisting motor is not driven. In hoisting operation the brake torque is nearly double the nominal torque of the motor. If the brake is not released e.g. when the nominal load of the hoisting apparatus is lowered, torque corresponding to the nominal torque is required of the hoisting motor for implementing the lowering movement. The hoisting motor can easily generate this torque. In that case the thermal losses in the brake are double the nominal power of the hoisting motor. This normally damages the brake after a drive of a few seconds, and when the drive is finished, the brake no longer holds the load in the air but it falls freely to the ground. Controlling of release of the hoisting motor brake is thus important for safety reasons. If the brake stays on, the motor winding may also burn, which causes considerable economical losses.
FR 2 675 790 discloses a solution for controlling release of the hoisting motor brake where an inductive sensor is used to detect brake release on the basis of the movement of the brake disc when the hoisting motor is started up. If no signal confirming brake release is received from the sensor, the use of the hoisting motor is interrupted and the alarm is activated. This solution is relatively complicated and unreliable in practice because it is difficult for the sensor to detect movement of the brake disc due to its short travel. Furthermore, the sensor and its installation increase the costs considerably.
U.S. Pat. No. 4,733,148 discloses a solution for controlling the brake of a printing press drive motor when the motor is started up. The solution comprises two phases: in the first phase it is checked that the brake torque is sufficient for preventing rotation of the motor when the brake is on. In the second phase it is checked that the brake has been released when the motor is started up. Checking of the sufficient capacity of the brake and brake release is based on determination of the rotational speed of the motor. The rotational speed of the motor is measured with a tachometer or determined from the armature voltage of the motor if the motor is a direct-current motor. Also in this solution both acquisition and installation of additional sensors increase the costs considerably.
The object of the present invention is to provide a new method and apparatus for controlling release of a hoisting motor brake in a hoisting apparatus.
The method according to the invention is characterized by measuring the current and supply voltage of the hoisting motor after the startup period of the hoisting motor and determining a variable which describes the load of the hoisting apparatus from the current and supply voltage and is compared with a pre-determined limit value, and interrupting hoisting or lowering of the load if the variable describing the hoisting apparatus load exceeds the limit value set for the hoisting movement when the load is hoisted or if it exceeds the limit value set for the lowering movement when the load is lowered.
The apparatus according to the invention is characterized in that the apparatus comprises means for measuring the current and supply voltage of the hoisting motor and a brake controlling device, which comprises means for determining a variable which describes the load of the hoisting motor from the current and supply voltage of the hoisting motor, and that the brake controlling device further comprises means for comparing the variable describing the load with a pre-determined limit value and means for interrupting hoisting or lowering of the load if the variable describing the hoisting apparatus load exceeds the limit value set for the hoisting movement when the load is hoisted or if it exceeds the limit value set for the lowering movement when the load is lowered.
The basic idea of the invention is that in a hoisting apparatus where electricity is used as the driving force and a squirrel cage motor as the hoisting motor for hoisting or lowering a load attached to the hoisting member of the hoisting apparatus, release of the hoisting motor brake is controlled by comparing a variable which describes the hoisting apparatus load and is determined from the current and supply voltage measured from the hoisting motor after its start-up period with a pre-determined limit value. If the variable describing the hoisting apparatus load exceeds a limit value set for the hoisting movement when the load is being hoisted or the limit value set for the lowering movement when the load is being lowered, the hoisting or lowering of the load is interrupted. According to a preferred embodiment of the invention, air gap torque is used as the variable describing the hoisting apparatus load. The air gap torque is preferably determined using magnetization flux of the hoisting motor.
An advantage of the invention is that release of the hoisting apparatus brake can be controlled without providing the brake with separate sensors or switches, the acquisition and installation of which increase the costs considerably and the function of which is unreliable due to short travels in the disc brake. The solution of the invention also improves thermal protection of the motor in the case of overloading and jamming of the rotor, which may result from improper use or malfunction of the hoisting apparatus. The method is also very accurate and reliable in varying operating conditions typical of hoisting operation if the air gap torque of the hoisting motor, which is determined from magnetization flux of the hoisting motor, is used as the variable describing the hoisting motor load.